WASHINGTON — President Obama on Tuesday night outlined to a deeply skeptical American public why he believes the United States may need to launch military strikes against Syria, even as he expressed hope that he could find a diplomatic solution to eliminating the regime’s chemical stockpile.

After originally planning to use the speech to urge Congress to approve a resolution authorizing the use of force — a measure that increasingly seemed unlikely to pass — Obama instead seized upon a potentially fragile offer from Russia that Syria’s chemical weapons be put under international control.

Citing “encouraging signs” of a diplomatic breakthrough, the president said he would talk directly with Russian President Vladimir Putin, a key ally of Syrian President Bashar Assad, about finalizing a deal to turn over stockpiles of chemical weapons. The president also announced he was sending Secretary of State John F. Kerry to Geneva to discuss the plan with Russia’s foreign minister.

Yet Obama spent the first three-quarters of the 15-minute nationally televised address laying out the necessity for a targeted strike to discourage the use of chemical weapons by Assad’s forces. In doing so, the president described heart-wrenching images of a suspected gas attack on families on Aug. 21, an attack that US intelligence agencies estimate killed more than 1,400 Syrians.

“When dictators commit atrocities they depend upon the world to look the other way until those horrifying pictures fade from memory,” Obama said. “What happened to those people, to those children, is not only a violation of international law, it is also a danger to our security.”

In a methodical, point-by-point approach, Obama sought to address doubts over the possible attack. In doing so, he used a technique he has consistently employed before, referring to correspondence he received from everyday citizens worried about the repercussions of military action.

He also urged Americans, in an emotional appeal, to watch the videos taken after the gas attack, which included images of children dead and dying.

Obama underscored his political challenge as he sought to reassure politicians both on the left and the right — as well as those in the middle — that Assad’s alleged use of chemical weapons could not go unpunished. He said that without serious consequences, other enemies will feel emboldened to use such weapons and possibly transfer them to terrorist organizations. He also argued that Syria’s neighbors, including American allies Turkey and Israel, would be put at risk.

“Sometimes resolutions and statements of condemnation are simply not enough,” he said. “What kind of world will we live in if the United States of America sees a dictator brazenly violate international law with poison gas and we choose to look the other way?”

Obama’s address to the nation was scheduled before Russia latched on to a suggestion by Kerry that a US strike could be avoided if international inspectors took control of Syria’s chemical weapons. With diplomacy taking center stage, the impetus faded to hold a vote this week on Obama’s request for congressional authorization to strike Syria.

As a result, Obama modified his address, and the United States worked with allies and the United Nations to explore a Russian plan that would allow the international community to take control of Syria’s chemical weapons stockpile. That decision earlier Tuesday followed calls Obama made to President Francois Hollande of France and Prime Minister David Cameron of Great Britain.

Also, earlier Tuesday, Syria signaled its willingness to sign on to the Russian proposal. Foreign Minister Walid al-Moualem said in an interview with Al-Mayadeen TV, an Arab satellite channel, that the government had agreed to acknowledge it had chemical weapons and would put them under UN control under the agreement with Russia.

But that plan was far from certain. Obama earlier Tuesday expressed doubts in a closed-door meeting with senators. And Russian authorities said they would refuse an effort by US and French officials for a UN resolution that would outline severe consequences for Syria if it failed to turn over its weapons. Putin said the United States should renounce force.

Obama directly addressed Congress, urging members to cast aside some of their deep reservations about beginning a new military engagement.

“To my friends on the right, I ask you to reconcile your commitment to America’s military might with a failure to act when a cause is so plainly just,” he said. “To my friends on the left, I ask you to reconcile your belief in freedom and dignity for all people with those images of children writhing in pain and going still on the cold hospital floor.”

It continued the stutter-step approach on how to handle Syria. The White House, largely through Kerry, had spent nearly a week outlining the case for strikes on Syria but then quickly pulled back when Obama decided he wanted to first get congressional authorization.

The administration last week launched an aggressive campaign to try to persuade a war-weary Congress and a skeptical international community to jump aboard. But that quickly began to shift on Monday, with a new Russian offer to avert military strikes.

The administration has been working on a two-track path: keep up the potential threat of military strikes, while searching for a diplomatic solution.

Obama asked senators to continue delaying their vote, saying he wanted more time to explore a diplomatic solution. But administration officials also want to continue putting the pressure on for potential strikes, saying it would strengthen their hand during negotiations.

“Nothing focuses the mind like the prospect of a hanging,” Kerry said during congressional testimony. “Well, it is the credible threat of force that has been on the table these last two weeks that has for the first time brought the regime to even acknowledge that they have a chemical weapons arsenal.”

As the Obama administration recalibrates its strategy, Congress is also shifting, with support diminishing across the political spectrum.

Senator Edward J. Markey, the Massachusetts Democrat who won the seat Kerry occupied for nearly three decades, said Tuesday before the address that he opposed authorizing military force.

“A vital national security risk is clearly not at play,” the Kentucky Republican said. “There are just too many unanswered questions about our long-term strategy in Syria, including the fact that this proposal is utterly detached from a wider strategy to end the civil war there.”

Senate majority leader Harry Reid said he would delay the vote to see what happens with the diplomatic negotiations.

“It’s important we do this well, not quickly,” he said. “We have to see what goes on. The last 24 hours have [shown] remarkable changes in what people are talking about.”

Meanwhile, a bipartisan group of eight senators, led by Republican John McCain of Arizona, began working on a resolution Monday afternoon that would back up diplomatic efforts with the threat of strikes.

The senators’ plan would set a deadline for the UN to vote on a resolution confirming that Syria used chemical weapons and would call on the UN to remove those weapons. The plan would authorize a strike if a deadline for UN action — the timing of which has not been worked out — is not met.

McCain said the United States should continue efforts on the diplomatic front, but was doubtful they would work.

“If you can get a result, then obviously it’s something that all of us can support, but I’m very, very skeptical,” he said. “But that doesn’t mean that we should reject out of hand any action that will call for securing these chemical weapons stocks.”

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